Method for matching surface texture and color of concrete roofing tiles

ABSTRACT

A method for changing the color and texture of ceramic roofing tiles, including the steps of identifying a damaged tile to be replaced, cleaning the damaged tile, optically measuring the damaged tile to yield a color determination, and selecting a replacement tile of the same shape and general color, immersing the replacement tile in a quantity of acid, removing the tile from the acid after a first predetermined length of time has passed to yield an acid-treated replacement tile, rinsing the acid-treated replacement tile to remove remaining acid, measuring the color of the acid-treated replacement tile, and washing the acid-treated replacement tile.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims priority to co-pending U.S. provisional patent application Ser. No. 63/390,093, filed on Jul. 18, 2022, which is incorporated by reference herein in its entirety.

TECHNICAL FIELD

The present novel technology relates generally to the field of ceramic engineering and, more particularly, to a method and apparatus of artificially weathering new ceramic roofing tile to match existing aged roofing tile.

BACKGROUND

Concrete roof tiles change with time, through a process commonly referred to as “aging”, due to the reaction of the concrete surface with rain, sunlight, and other environmental factors. This aging process is essentially a corrosion process that alters the surface texture of the tile as well as its color. The consequence of this alteration is that if a tile needs to be replaced, a new tile of the same style and color no longer matches the remaining original tiles, thus limiting the ability to replace damaged tiles, and usually results in the replacement of the entire roof. Replacing a roof is costly in terms of money and in terms of CO₂— the average concrete roof tile emits three pounds of CO₂ per tile due to the CO₂ cost of making concrete. If an entire roof requires replacements, this amounts to tons of CO₂ added to the environment. Thus, there remains a need for preparing new tiles that match the color and texture of the weathered or aged existing original tiles. The present novel technology addresses this need.

SUMMARY

The instant novel technology relates to a method to artificially age a new concrete roof tile to create both similar color and surface texture to appear similar to a tile that was naturally aged that was installed on a roof. Color and texture manipulation is accomplished using a variety of acids including primarily nitric acid (HNO₃) or sulfuric acid (H₂SO₄). Other acids, such as hydrochloric acid (HCl, also commonly known as muriatic acid), phosphoric acid (H₃PO₄), or citric acid (HOC(CO₂H)(CH₂CO₂H)₂) can also be used but are less effective at altering the color and texture. The use of mineral bases, such as sodium hydroxide (NaOH), potassium hydroxide (KOH), or ammonium hydroxide (NH₄OH), were essentially ineffective at altering the color or surface of the concrete tile. Tiles treated to alter the color and texture do not exhibit a significant reduction in strength compared to aged tiles, with strengths remaining well within the certification strength limit. Thus, the properties and appearance are sufficiently similar to aged tiles that treated tiles can be used to replace damaged or broken concrete tiles on an existing roof. The specific application for treating a potential replacement concrete tile is to (1) identify the make of the existing roof tiles (color, shape or form, manufacturer, etc.); (2) find a candidate (similar base color, identical form and manufacturer); (3) establish the treatment protocol (based on the matrix provided in the patent); (4) treat the new tile using the appropriate acid bath with the prescribed treatment protocol; (5) measure the color and demonstrate that the color range is similar to the color range on existing roof; (6) replace the damaged roof tiles. To facilitate the handling of the tiles in the acid bath a device was constructed that will treat four tiles at once.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A graphically illustrates the range of a* (red [+] to green [−]) and b* (yellow [+] to blue [−]) for 80 measured concrete roof tiles (both new and used).

FIG. 1B graphically illustrates the range of whiteness L* from black [0] to white [100]) for the same sample of 80 tiles showing that whiteness also varies but that the scatter in the data decreases as the tile becomes more red (higher a*).

FIG. 2 . is a schematic illustration of the color change matrix using 0.2N nitric acid and 0.3N sulfuric acid.

FIG. 3 is a general process flowchart for the treatment of concrete roof tiles to match the color and texture of aged roof tiles obtained from an installed roof.

FIG. 4 graphically illustrates examples of four new tiles (bottom row) treated to match four aged tiles (top row) using the procedures described within.

FIGS. 5A and 5B graphically illustrate the color shift for red, green, and grey tiles, after treatment with a 1.0N nitric acid solution for 60 minutes, as illustrated by the measured color values, plotted as (A) b* as a function of a* and as (B) a* versus L*.

FIGS. 6A and 6B graphically illustrate color shift for green, red, and tan tiles, after treatment with a 0.6N nitric acid solution for 60 minutes, followed by a 0.4N sulfuric acid solution (and finally with a 0.1N citric acid solution).

FIG. 7 is a process flowchart for the treatment of new red concrete roof tiles to match aged red concrete roof tiles.

FIGS. 8A and 8B graphically illustrate color measurement results for red tiles treated with a 0.9N nitric acid solution for 60 minutes.

FIG. 9 is a process flowchart for the treatment of new green concrete roof tiles to match aged green concrete roof tiles.

FIGS. 10A and 10B graphically illustrate color measurement results for green tiles treated with a 0.7N nitric acid solution for 60 minutes.

FIG. 11 is a process flowchart for the treatment of new tan concrete roof tiles to match aged tan concrete roof tiles.

DETAILED DESCRIPTION

For the purposes of promoting an understanding of the principles of the novel technology and presenting its currently understood best mode of operation, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the novel technology is thereby intended, with such alterations and further modifications in the illustrated device and such further applications of the principles of the novel technology as illustrated therein being contemplated as would normally occur to one skilled in the art to which the novel technology relates.

As used herein, a “new concrete roof tile” or “new tile” denotes a tile that has not been installed on a roof. A “used concrete roof tile” or “used tile” or “aged tile” indicates a tile that has been installed on a roof but was removed.

The present novel technology relates to a method to artificially age a new tile to create both a similar surface texture and color to match a tile that has been previously installed on a roof and allowed to weather and age. The surface texture is matched in a relative manner, with the surface reflectance providing an indication of the surface texture, to the point of being difficult to discern roughness differences by eye. Color is matched using a scientific measure of color using the internationally agreed upon 3-parameter International Commission on Illumination (CIE) system of L*, a*, and b*, wherein L* denotes perceptual lightness, a* is the green-magenta opponent color axis, and b* is the blue-yellow opponent color axis.

New concrete roof tiles are often treated with a sealant, usually as a step in the tile manufacturing process, that is designed to increase the resistance of the exposed tile surface to corrosion and to extend the tile lifetime and to keep the original appearance of the tile for as long as possible. The sealant is typically an organic coating. However, the organic coating appears to deteriorate with time, possibly due to exposure to sunlight and other elements.

Concrete roof tiles are typically composed of a mixture of Portland cement (a calcium silicate of commercial origin), sand (usually quartz), and a pigment (composed of a variety of chemicals including iron oxides, chromium oxide, manganese oxide, carbon, copper oxide, and the like). Sand constitutes the largest portion of the mixture, followed by Portland cement, and finally pigment. Since the pigment is mixed with the sand and Portland cement and then with water, the pigment is distributed uniformly throughout the tile. It is common, however, for a used tile to have a different color on the back when compared to the front due to aging.

It should also be noted that concrete roof tiles are rarely, if ever, a single color. Even within a single roof tile there is a color range. To alter a new tile, or a used tile, to match the tile on a roof, it is not necessary to match the color exactly, but to provide a spectrum of colors that match the existing roof but also to provide a color variation or palette that is similar to the color variation exhibited by the existing roof. As shown in FIG. 1 , concrete roof tiles have a broad range of color, as measured using L*, a*, and b*.

The natural aging process that a concrete roof tile experiences on a roof corrodes the surface of the tile, stripping away the sealant and attacking the cement structure, usually through the chemical dissolution, or corrosion, of the Portland cement component.

Sealant Removal of New Tiles.

The sealant is an organic coating that is typically difficult to remove on a new tile. Several commercial solvents, usually referred to as “strippers,” work to varying degrees to remove the sealant. The other aspect of the removal of the sealant from a new tile using a stripper is that the surface texture remains unaffected so that the surface texture after sealant removal is similar to the surface texture of a new tile. However, the acid treatment process as described below removes the sealant effectively, and therefore generally eliminates the need for a separate sealant removal step.

Modification of Tile Color.

Commercial concrete roof tiles cover a broad range of the color spectrum, ranging from about 25 to 65 for L*, from about −5 to +25 for a*, and from about −3 to +27 for b*. L* ranges from 0 to 100 with 100 being pure white and 0 being pure black. The a* axis has an open-ended range with red being positive and green being negative a* values. Similarly, b* is also an open-ended scale with yellow being positive and blue represented by a negative b* value. As anyone who has purchased wall paint in the past 70 years knows, any color is a mixture of pigments and there are few “pure colors”—the same is true of concrete roof tiles.

To alter the color, as well as the tile surface texture, a new tile is soaked in a dilute acid solution. The acids are typically nitric acid (HNO₃) and/or sulfuric acid (H₂SO₄). Hydrochloric acid (HCl, or muriatic acid) may also be used, but does not significantly affect the color, but is generally quite aggressive and can potentially cause unnecessary damage to the tile surface. Phosphoric acid (H₃PO₄) and citric acid (HOC(CO₂H)(CH₂CO₂H)₂) are generally less effective at altering the color or texture of a concrete tile, while citric acid is effective at removing hard water and calcium deposits and can be used to tune the apparent color of a concrete tile after primary acid treatment. The use of citric acid darkens the tile color by dissolving white deposits (assumed to be calcium carbonate, CaCO₃, or calcium sulfate, CaSO₄) that form by the reaction of the cement with rainwater and the atmosphere. The treatment of cement roof tiles in base solutions—sodium hydroxide (NaOH), potassium hydroxide (KOH), and ammonium hydroxide (NH₄OH)— is generally less effective at altering either the color or the texture of the concrete roof tile.

Acid concentrations are typically low, in the range of 0.5-1.5 Normal (commonly denoted 0.5N to 1.5N), but higher or lower acid concentrations may also work. (For reference, white vinegar is typically about 0.8N.) Higher concentrations, however, tend to result in structural damage to the tile in short periods of time and are more difficult to control. Lower acid concentrations lengthen the time required to alter color and texture, eventually becoming infeasible below 0.2N. As illustrated schematically in FIG. 2 , nitric acid increases a* and b* values, while leaving L* approximately constant, whereas sulfuric acid increases L* while reducing a* and b*. In contrast, citric acid alters only L*, leaving a* and b* unchanged. The various acids do not behave the same and most notably, sulfuric acid tends to make the treated tile more white, due to the deposit of white calcium salts (CaSO₄) that precipitate from solution on the surface of the tile. As noted, sulfuric acid also changes the level of red and blue. The magnitude of the color shift was also not the same for nitric acid or sulfuric acid, nor for the initial tile color (generally grouped as red, tan, green, and grey), but the changes observed were generally similar to the color shift observed by weathering of the tile on a roof. Therefore, to achieve a specific color it may be advantageous to have more than one process step, altering the color in one direction then recover a portion of that color shift to obtain a target color.

It was observed that the treatment of a concrete roof tile in a dilute acid solution is generally complete after about one hour. Longer times alter the color only minimally but can lead to structural damage to the tile in a manner similar to that observed by treatment at higher acid concentration levels. Similarly, treatment with lower acid concentrations required significantly longer times to effect color and/or texture changes. So, within these operating conditions, the acid concentration is a more preferred control variable than the time in the acid bath, although either or both may be used to control the process. After the chemical treatment, the tiles are immersed in water to remove the treatment chemicals. The so-treated tiles are then dried and re-sealed if desired.

Thus, to match a specific color it is necessary to obtain a candidate tile that can be treated to match that aged concrete tile color target. It should be obvious that a green tile cannot be altered to match a red tile. But a new red tile that is not the same color as a used red tile can be altered to obtain a similar red color if the candidate tile is reasonably within a prescribed “distance” from the target. This is generally not a problem, however, as the color of new concrete roof tiles has wide variability, so finding a new tile within the right color range is feasible.

To identify candidate tiles, color measurements are taken of the tile inventory. The color measurement can be obtained from the front of the tile or the back—these are generally similar. If a used tile is evaluated, the color should be measured on the band at the top of the tile that was shielded from sunlight and the elements by the overlap of the tile above. This is a region at the top of a tile about three inches in width. The target used tile (obtained from the roof) is then measured and a candidate tile is identified. The candidate tile can be a new tile or a used tile but generally a new tile is preferred. All color measurements are obtained using a spectrophotometer and commercial software. 10 points are measured on each tile using a specific predetermined pattern for reproducibility.

Modification of Tile Texture and Color.

The use of specified acid concentration range serves three purposes: (1) this acid concentration range removes the sealant (if present); (2) this acid concentration range alters the surface texture of the tile; and (3) this acid concentration range shifts the tile color in a manner similar to natural aging on a roof.

Aged cement roof tiles have a surface texture that is significantly rougher than a new tile, frequently to the point of exposing aggregate particles embedded in the cement matrix. Aggregate particles tend to be lighter in color and are immediately obvious when observed closely, with the consequence of causing an increase in the measured whiteness of the tile (L* increases). To produce a tile of a similar texture it is necessary to corrode the tile surface using a higher concentration of acid or base, typically in the range of 0.5-1.5 N. (Of course, higher acid concentration will also expose aggregate.)

Once a new tile has been treated using an acid solution the surface texture is altered to be similar to that of an aged tile. Further treatments are then possible, if necessary, to match the color range of a target tile.

Unlike color measurement, there does not appear to be a reliable method to quantify surface roughness. Concrete is an intrinsically rough surface and exceeds the measurement abilities of most roughness measurement systems, such as profilometer methods or gloss quality measurements (as obtained from a gloss meter used for paint and ceramic glazes). The surface roughness assessment has, therefore, been qualitative.

A tile measurement and treatment protocol for color matching was developed to provide a balance between time efficiency and workload, using the process outlined in FIG. 3 . The aged concrete roof tiles are first cleaned of dirt and debris, such as by using a pressure washer. The upper portion of the tile (which was protected from weathering) typically exhibits hard water deposits which are not present on the lower portion (which was weathered). The upper portion is then washed with a 1.0 N citric acid solution to remove hard water deposits and rinsed with tap water and dried. The color of dried tiles is measured optically using a spectrophotometer. New tiles were selected to match the shape profile of the used tile (i.e., style and manufacturer), and with colors within the adjustment range of the upper shaded portion of the aged tile to obtain the color of the unshaded aged cement tile surface (that typically has been subject to prolonged exposure to sunlight and the elements).

New tiles are treated in the acid bath to remove any sealant and to provide an initial color shift and texture alteration. After removal from the treatment bath, tiles are rinsed in a tap water bath to remove reaction products and any residual nitric acid, then moved to a wash tank and pressure washed to remove any additional residue and loose aggregate. Next, tiles are dried then measured with the spectrophotometer to determine whether additional treatment is needed. Examples of four tiles that were treated to match aged tiles removed from a roof is shown in FIG. 4 .

If the color and texture after initial treatment is not a sufficient match to the exposed portion of the used tile, the tiles are further treated using one or more of the following methods:

-   -   1. Tiles can be immersed in a 1.0 N sulfuric acid solution for         one hour to provide effective color shifts without additionally         affecting surface texture. Sulfuric acid increases L* (more         white), slightly increases a* (more red) and decreases b* (more         blue).     -   2. Tiles can be immersed in a 1.0 N hydrochloric acid solution         for 10 minutes to increase surface roughness by exposing more         aggregate without affecting color.     -   3. 1.0 N citric acid solution can be applied and allowed to         react for ten minutes followed by rinsing with tap water. Citric         acid decreases L* (less white) without altering a* or b*.     -   4. In all cases, tiles are rinsed post treatment with tap water,         pressure washed, then dried.

Nitric acid increases a* and b*, with small increases in L* (whiteness) attributed to exposing aggregate. Initial trials of the process using nitric acid concentrations in the range of 0.2-0.3 N were effective at producing appropriate color shifts and surface texture changes with a treatment time of 24 hours. Nitric acid concentrations of 2.0 N or greater demonstrated excessive surface damage to the tiles. Within the range between 1.5-2.0 N, treatment time for consistent results is typically a function of treatment time, and thus creates the potential for excessive surface damage. Within the range of 0.4-1.5 N, treatment time of one hour provided effective sealant removal and consistent surface texture and color changes with no need to precisely adjust treatment time, even as acid concentration was depleted with successive cycles. FIG. 5 shows the color shift associated with treatment in nitric acid (1.0N for 60 minutes), illustrating that the nitric acid treatment alters the color of red, green, and grey tiles.

Using sulfuric acid for initial treatment is typically less effective than nitric acid due to incomplete removal of the sealant. Like the nitric acid bath treatments, sulfuric acid concentrations above 1.0 N were effective, but treatment time was variable depending on small changes in acid concentration, with concentrations between 0.4-1.5 N and a treatment time of one hour providing consistent effects. However, the incomplete removal of sealant results in uneven surface texture and color changes. But when tiles are treated with nitric acid, followed by treatment with a dilute sulfuric acid solution, and then with citric acid (due to precipitation on the surface increasing the whiteness), the color shift is reasonable for red and tan tile (with almost no color shift in green tiles) thus offering a smaller color shift than observed by nitric acid alone, as shown in FIG. 6 .

Summary of Ineffective Treatments

-   -   1. Hydrochloric acid alone is not an effective treatment because         it removes sealant and provides changes to surface texture by         exposing aggregate but does not alter color.     -   2. Citric acid alone is not an effective treatment because it         does not remove sealant during initial treatment.     -   3. Phosphoric acid and acetic acid may be used in place of         citric acid but do not remove sealant during initial treatment         and did not produce surface texture changes.     -   4. Sodium hydroxide, potassium hydroxide, and ammonium hydroxide         treatments were ineffective at removing sealant, altering         texture, or adjusting color.

Example 1

A red concrete roof tile, removed from a roof (naturally weathered) was pressure washed and dried. 1.0 N citric acid was applied to the protected portion of the tile to remove hard water deposits for 10 minutes, then the tile was rinsed in clean water and dried. The color of the weathered tile was measured in two regions: (1) the upper covered region, to provide an indication of which new tile to pick for acid treatment, and (2) measured in the lower, exposed surface, to provide a target color for alteration. The candidate tile was then immersed in a nitric acid solution with a concentration of 0.9 N for 60 minutes, rinsed in a clean water bath to neutralize the acid, and pressure washed. After washing, the tile was dried, and the color measured to verify a color match. The process is outlined in FIG. 7 and the color measurement results are presented in FIG. 8 .

Example 2

A green concrete roof tile, removed from a roof (naturally weathered) was pressure washed and dried. A 1.0 N citric acid solution was applied to the protected portion of the tile to remove hard water deposits for 10 minutes, then the tile was rinsed in clean water and dried. The color of the weathered tile was measured in two regions: (1) the upper covered region, to provide an indication of which new tile to pick for acid treatment, and (2) measured in the lower, exposed surface, to provide a target color for alteration. The candidate tile was then immersed in a nitric acid solution with a concentration of 0.7 N for 60 minutes, rinsed in a clean water bath to neutralize the acid, and pressure washed. After washing, the tile was dried, and the color measured. The tile was then immersed in a hydrochloric acid solution with a concentration of 1.0 N for 10 minutes to increase surface texture by exposing more aggregate. A process flowchart is shown in FIG. 9 . After washing, the tile was dried, and color measured to verify a color match, as illustrated in FIG. 10 .

Example 3

A tan concrete roof tile, removed from a roof (naturally weathered) was pressure washed and dried. 1.0 N citric acid was applied to the protected portion of the tile to remove hard water deposits for 10 minutes, then the tile was rinsed in clean water and dried. The color of the weathered tile was measured in two regions: (1) the upper covered region, to provide an indication of which new tile to pick for acid treatment, and (2) measured in the lower, exposed surface, to provide a target color for alteration. The candidate tile was then immersed in a nitric acid solution with a concentration of 0.6 N for 60 minutes, rinsed in a clean water bath to neutralize the acid pressure washed. After washing, the tile was dried, and the color measured. The tile was immersed in a sulfuric acid solution with a concentration of 0.4 N, rinsed in a clean water bath to neutralize the acid, and pressure washed. After drying, the tile was color measured. The tile was lighter in color than the target, so the tile was treated with a 1.0N solution of citric acid for 10 minutes then washed. The process flowchart for the treatment of tan concrete roof tiles is shown in FIG. 11 . The tan tile matched the color and texture of the aged tile, after drying, as shown in FIG. 4 .

TABLE 1 (for FIG. 8) - Color measurements for Red concrete roof tile. New Red Aged Red Acid treated Red L* a* b* L* a* b* L* a* b* 44.48 11.18 11.60 43.72 11.37 10.95 45.75 10.09 11.30 43.62 10.92 11.16 45.26 9.99 9.88 45.80 9.59 10.77 42.92 11.20 12.02 47.19 8.26 10.29 46.75 10.05 11.21 43.57 11.48 12.58 42.27 10.85 10.20 46.72 9.17 10.81 44.14 12.04 12.22 45.93 9.02 9.56 45.64 9.29 10.26 41.94 10.70 11.45 45.27 9.56 10.28 45.85 10.10 11.01 45.10 11.92 12.32 43.51 9.72 9.64 45.38 7.97 9.81 42.55 10.12 11.09 45.25 9.87 9.87 44.04 8.74 8.53 43.13 10.58 12.08 44.49 9.86 9.56 44.67 9.37 11.45 44.62 10.24 12.47 45.86 11.59 12.16 44.00 10.20 10.71 ±0.994 ±0.653 ±0.538 ±1.415 ±1.023 ±0.799 ±0.967 ±0.713 ±0.873

TABLE 2 (for FIG. 10) - Color measurements for Green concrete roof tile. New Green Aged Green L* a* b* L* a* b* 53.35 −1.29 7.50 58.40 −4.19 7.31 53.07 −1.96 9.88 60.34 −5.13 9.39 52.62 2.11 10.36 55.67 −3.42 6.25 54.47 −2.32 10.00 56.13 −3.70 6.11 49.88 −2.31 9.18 58.98 −3.77 7.00 51.61 −2.20 10.16 59.43 −4.33 7.36 51.41 −2.36 10.40 58.89 −4.94 9.09 53.56 −0.87 11.10 57.82 −3.56 6.00 51.16 −2.06 10.32 57.66 −3.80 6.49 53.89 −2.91 9.32 55.78 −3.55 6.35 ±1.439 ±1.430 ±0.984 ±1.611 ±0.597 ±1.209 Nitric Acid Treated Green HCl Treated Green L* a* b* L* a* b* 57.77 −2.28 6.30 52.31 −1.67 7.03 56.78 −2.16 7.47 52.46 −1.96 7.34 55.27 −2.32 6.74 54.27 −2.15 6.19 54.22 −1.36 5.89 48.03 −2.35 9.25 54.42 −1.88 6.94 56.20 −2.18 7.72 53.40 −1.74 5.53 54.74 −2.44 7.05 59.03 −2.07 6.96 57.10 −2.19 7.51 56.54 −1.71 7.57 49.43 −2.31 8.49 56.67 −1.64 7.34 53.56 −2.06 6.58 54.68 −2.01 6.44 52.31 −2.21 6.51 ±1.771 ±0.307 ±0.678 ±2.802 ±0.219 ±0.937

While the novel technology has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character. It is understood that the embodiments have been shown and described in the foregoing specification in satisfaction of the best mode and enablement requirements. It is understood that one of ordinary skill in the art could readily make a nigh-infinite number of insubstantial changes and modifications to the above-described embodiments and that it would be impractical to attempt to describe all such embodiment variations in the present specification. Accordingly, it is understood that all changes and modifications that come within the spirit of the novel technology are desired to be protected. 

We claim:
 1. A method for changing the color and texture of ceramic roofing tiles, comprising: a) identifying a damaged tile to be replaced; b) cleaning the damaged tile; c) optically measuring the damaged tile to yield a color determination; d) selecting a replacement tile of the same shape and general color; e) stripping sealant from the replacement tile; f) immersing the replacement tile in a quantity of nitric acid; g) removing the tile from the acid after a first predetermined length of time has passed to yield an acid-treated replacement tile; h) rinsing the acid-treated replacement tile to remove remaining acid; i) drying the tile; and j) measuring the color of the acid-treated replacement tile.
 2. The method of claim 1 and further comprising: k) after b) and before c), applying citric acid to an unweathered portion of the damaged tile.
 3. The method of claim 1 and further comprising: l) after i) and before j), adjusting the color of the acid-treated replacement tiles via immersion in a second acid for a second predetermined period of time; wherein the second acid is selected from the group consisting of sulfuric acid and hydrochloric acid; and wherein the second acid has a concentration of between 0.5N and 1.5N.
 4. The method of claim 1 wherein the first predetermined period of time is one hour and wherein the nitric acid has a concentration of between 0.5N and 1.5N.
 5. The method of claim 1 wherein during step c), the damaged tile is assigned first L*, a*, and b* values; and wherein during step i), the acid-treated replacement tile is assigned second L*, a*, and b* values.
 6. The method of claim 5 and further comprising the step of: l) comparing first and second sets of L*, a*, and b* values.
 7. A color matched tile for replacing a weathered tile, comprising: an acid-etched exterior portion; and an interior portion disposed beneath the acid-etched exterior portion; wherein the acid-etched exterior portion has substantially the same L*, a*, and b* values as does the weathered tile.
 8. The color matched tile of claim 7 wherein the acid-etched tile is substantially free of sealant.
 9. The color matched tile of claim 7 wherein the acid-etched tile has substantially the same surface texture as the weathered tile.
 10. A process for matching the color and texture of a new ceramic roofing tile to a weathered tile, comprising: m) identifying a weathered tile to be replaced; n) cleaning the weathered tile; o) treating an unweathered portion of the weathered tile with citric acid; p) optically measuring the weathered tile to yield a color determination; q) selecting a replacement tile of the same shape as the unweathered tile and the same color as the unweathered portion of the weathered tile; r) stripping sealant from the replacement tile; s) immersing the replacement tile in a bath of nitric acid; t) removing the tile from the bath after a first predetermined period of time to yield an acid-treated replacement tile; u) removing residual acid from the acid-treated replacement tile; v) adjusting the color of the acid-treated replacement tiles via immersion in a second acid for a second predetermined period of time; w) drying the acid-treated replacement tile; and x) measuring the color of the acid-treated replacement tile; wherein the first predetermined period of time is one hour; wherein the nitric acid has a concentration of between 0.5N and 1.5N; wherein the second acid is selected from the group consisting of sulfuric acid and hydrochloric acid; and wherein the second acid has a concentration of between 0.5N and 1.5N.
 11. The method of claim 10 wherein during step p), the weathered tile is assigned first L*, a*, and b* values; and wherein during step x), the acid-treated replacement tile is assigned second L*, a*, and b* values.
 12. The method of claim 11 and further comprising the step of: y) comparing first and second sets of L*, a*, and b* values.
 13. The method of claim 10 wherein the step s) has a duration of one hour. 